1. Field of the Invention
The present invention relates a to web driving device for cleaning an object or for applying liquid such as oil to an object by the action of contact. The web driving device is equipped in a device such as an image forming apparatus (e.g., a copier, a facsimile, or a printer). More specifically, the web driving device is equipped in a fixing device or a photosensitive device in the image forming apparatus.
2. Discussion of the Background
A conventional web driving device, as depicted in FIG. 1, is equipped in a fixing device. The fixing device generally includes a fixing roller 1 that has a heater 3 inside such that a surface of the fixing roller 1 is heated and controlled at prescribed temperature. A press roller 2 presses the fixing roller 1 and makes a nip portion between them. When a sheet of paper 5 having unfixed toner image thereon is fed through the nip portion, the toner image is fixed to the paper 5. The fixing device further includes a web device having a web 44 made of an unwoven wiper that is constructed of aramid fiber mixed with PET (polyethylene terephthalate) fiber. The web 44 contains oil as required. One edge of the web 44 is fixed to a winding axis 42 and another edge of the web is fixed to a supplying axis 43. Most of the web 44 is wound in the supplying axis 43 in initial condition. A pressing roller 45 is located between the winding axis 42 and supplying axis 43 for pressing the web 44 to the fixing roller 1. The web 44 is moved step by step and then the web 44 removes a residual quantity on the fixing roller 1 or supplies oil to the fixing roller 1. A motor 72 drives the winding axis 42 for winding the web 44 at a constant angle in one action via a transferring mechanism 71 such as a gear or a timing pulley. The pressing roller 45 has a layer made of a silicon rubber or a forming rubber on a core metal.
Construction of the web driving device and amount of sending (or unwinding) of the web has numerous variations depending upon the manner in which the image forming apparatus is going to be utilized. The amount of sending is generally limited to a small amount because the web 44 is restricted in length or setting space. More specifically, the amount of sending is generally limited to between 0.5 mm/hour and 2 mm/hour.
In this conventional web driving device, there is a problem that when the web is pulled unexpectedly (for example during a paper jam) and the fixing roller is manually rotated via a handling knob, then the winding axis is rotated in conjunction with the fixing roller. The reason why the problem comes up is the winding axis is usually put only under light load for helping the supplying axis for taking up the web. More specifically, when a paper 5 is jammed, as depicted in FIG. 2(a), and the paper 5 is taken out along a normal feeding direction (indicated by an arrow), the web 44 may not be drawn by a torque of the fixing roller 1 as the winding axis 42 is locked by the motor 72 in an undriving condition. However, when a paper 5, as depicted in FIG. 2(b), is taken out along a counter direction of the normal feeding direction (indicated by an arrow), the web 44 is drawn by a torque of the fixing roller 1 (as shown using a dotted lines) because the supplying axis 43 is not locked. Accordingly, cleaning ability of the web 44 is reduced. Furthermore, the web 44 may be caught in the nip portion by the slack when the printing action restarts.
To solve the above problem, a device is disclosed in a Laid-Open Japanese Patent Application No. 08-185074. The device has a locking mechanism for preventing the looseness of the web. The device has a supplying axis including a ratchet gear at the edge thereof, a winding axis for taking up the web, a solenoid actuated in response to winding action of the winding axis.
The solenoid has a ratchet hook that is engaged with the ratchet gear only when the winding axis rotates. However, as a controller must control the action of the solenoid and the action of the winding axis accurately, controlling by the controller becomes difficult. Furthermore, the device must have a ratchet hook and a solenoid of increased rigidity in order to insure that the rotation of the supplying axis is properly stopped. Therefore the device must be upsized.